(1) Field of the Invention
The present invention relates to a silicon nitride sintered body having a high oxidation resistance and a high strength at a high temperature, and a process for the preparation of this silicon nitride sintered body.
(2) Description of the Related Art
Since a silicon nitride sintered body has a high strength, a high heat resistance, a high thermal shock resistance, a high abrasion resistance, and a high oxidation resistance, it is probable that this silicon nitride sintered body will be utilized as structural ceramics for gas turbine members, steel-making high-temperature rolls and the like where severe high-temperature conditions apply. Since it is difficult to sinter a material composed solely of silicon nitride, densification is attempted by adding various sintering aids. The sintering aids, however, form a glassy phase having a low melting point in the grain boundary of silicon nitride and the high-temperature strength is often reduced by the presence of this glassy phase. To eliminate this disadvantage, the following methods have heretofore been proposed.
(1) The addition of an oxide of a rare earth element, such as yttrium oxide.
(2) The addition of an oxide of a rare earth element, such as yttrium oxide, and an oxide such as aluminum oxide (see, for example, Japanese Examined Patent Publication No. 49-21,091).
(3) The addition of yttrium oxide, aluminum oxide and aluminum nitride, and after sintering, the precipitation of a second phase of yttrium aluminum garnet (YAG) or the like by a heat treatment (Japanese Unexamined Patent Publication No. 58-55,375).
However, these methods have the following problems from the practical viewpoint.
(1) According to the method in which yttrium oxide is added, the grain boundary of silicon nitride is bonded through, for example, a highly viscous glass or a crystalline composition such as Si.sub.3 N.sub.4 .cndot.Y.sub.2 O.sub.3 and, therefore, the reduction of the high-temperature strength and high-temperature creep resistance is small, but the high-temperature strength is still too low. Moreover, since sintering is difficult, pressureless sintering cannot be employed.
(2) According to the method in which yttrium oxide and aluminum oxide are added, sintering is promoted and a sintered body having a high density and a high strength can be obtained even by pressureless sintering. However, the reduction of the high-temperature strength is still large, and unless a special sintering method such as a hot-press method is adopted after an operation such as a crystallization treatment, a dense sintered having an excellent high-temperature strength cannot be obtained.
(3) According to the method in which a second phase is precipitated by a heat treatment after sintering, the reduction of the high-temperature strength is moderated, but the precipitation of the second phase causes a change of the volume and, therefore, the thermal shock resistance of the sintered body is lowered.
Moreover, in the sintered bodies obtained according to these methods, the temperature at which the oxidation resistance is effected is about 1,200.degree. C. at highest, and therefore, such sintered bodies cannot be applied at higher temperatures, and accordingly, silicon carbide is mainly used as the applicable ceramic material at temperatures higher than about 1,300.degree. C.